Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers
Authors: David N. Fisman, Raina Macintyre
Journal: Annals of Internal Medicine (2023)

LettersJuly 2023Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care WorkersFREEDavid N. Fisman, MD, MPH, Raina Macintyre, MBBS, MAppEpid, PhDDavid N. Fisman, MD, MPHUniversity of Toronto Dalla Lana School of Public Health, Toronto, Ontario, CanadaSearch for more papers by this author, Raina Macintyre, MBBS, MAppEpid, PhDKirby Institute, University of New South Wales, Kensington, New South Wales, AustraliaSearch for more papers by this authorAuthor, Article, and Disclosure Informationhttps://doi.org/10.7326/L23-0073 SectionsAboutVisual AbstractPDF ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinkedInRedditEmail TO THE EDITOR: Regarding their trial, Loeb and colleagues (1) state, “the overall estimates rule out a doubling in hazard”; however, no such conclusion is possible. The trial used an extraordinary threshold (a hazard ratio of 2, or a 100% relative increase in risk) for noninferiority and was underpowered to find smaller but still important risks. (We estimate that a 4-fold increase in sample size would have been needed to identify a 50% increase in relative hazard.) Power aside, design flaws biased the study toward the null result that was obtained.The intervention under study was incorrect use of N95 respirators—intermittently rather than continuously. SARS-CoV-2 is an airborne pathogen (2). Infection occurs via inhalation of shared air, and infective aerosols accumulate over time in closed indoor settings. As such, only continuous use of N95 respirators protects health care workers against respiratory infection; intermittent use of medical masks and respirators is equally ineffective (3). Unplanned crossover (those randomly assigned to medical masks could reassign themselves to the N95 group on the basis of unrecorded risk assessment) and contamination due to failure to use a cluster design further biased study results toward the null (4).Notwithstanding lack of power and multiple biases—and although only 21 infections developed among 301 participants recruited in Canada and Israel through May 2021—analysis according to the registered protocol reveals a doubling of risk for infection for medical masks (relative risk, 2.05 [95% CI, 0.85 to 4.95]; P = 0.10) in these participants. The study had come close to showing inferiority after recruiting only a fraction of its prespecified sample size. Around this time, the authors recalculated their required sample size (in July 2021) as 1010 participants and began recruiting participants in Pakistan at a site not mentioned in the trial's registered protocol. Six months later, recruitment in Pakistan was discontinued and was begun in Egypt (also not registered in the protocol). Final results were heavily influenced by the inclusion of the sites in Egypt, with more than 70% of infections originating there. As Altman and associates note, “when authors substitute other outcomes after the trial has started there must be concern that such changes were done with knowledge of the data. That casts doubt on the reliability and integrity of the results” (5).Lastly, the performance of this trial lacked equipoise in the face of clear engineering evidence of the superiority of respirators for airborne pathogens. The fact that this trial was done in a flawed manner that could not provide valid results means that participants were endangered for no reason.References1. Loeb M, Bartholomew A, Hashmi M, et al. Medical masks versus N95 respirators for preventing COVID-19 among health care workers. A randomized trial. Ann Intern Med. 2022;175:1629-1638. [PMID: 36442064] doi:10.7326/M22-1966 LinkGoogle Scholar2. Centers for Disease Control and Prevention. How COVID-19 spreads. 2022. Accessed at www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/how-covid-spreads.html on 8 December 2022. Google Scholar3. MacIntyre CR, Wang Q, Seale H, et al. A randomized clinical trial of three options for N95 respirators and medical masks in health workers. Am J Respir Crit Care Med. 2013;187:960-6. [PMID: 23413265] doi:10.1164/rccm.201207-1164OC CrossrefMedlineGoogle Scholar4. Chen X, Chughtai AA, MacIntyre CR. Herd protection effect of N95 respirators in healthcare workers. J Int Med Res. 2017;45:1760-1767. [PMID: 27789807] doi:10.1177/0300060516665491 CrossrefMedlineGoogle Scholar5. Altman DG, Moher D, Schulz KF. Harms of outcome switching in reports of randomised trials: CONSORT perspective. BMJ. 2017;356:j396. [PMID: 28196813] doi:10.1136/bmj.j396 CrossrefMedlineGoogle Scholar Comments0 CommentsSign In to Submit A Comment Author, Article, and Disclosure InformationAuthors: David N. Fisman, MD, MPH; Raina Macintyre, MBBS, MAppEpid, PhDAffiliations: University of Toronto Dalla Lana School of Public Health, Toronto, Ontario, CanadaKirby Institute, University of New South Wales, Kensington, New South Wales, AustraliaDisclosures: Disclosures can be viewed at www.acponline.org/authors/icmje/ConflictOfInterestForms.do?msNum=L23-0073. PreviousarticleNextarticle Advertisement FiguresReferencesRelatedDetailsSee AlsoMedical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers Mark Loeb , Amy Bartholomew , Madiha Hashmi , Wadea Tarhuni , Mohamed Hassany , Ilan Youngster , Ranjani Somayaji , Oscar Larios , Joseph Kim , Bayan Missaghi , Joseph V. Vayalumkal , Dominik Mertz , Zain Chagla , Maureen Cividino , Karim Ali , Sarah Mansour , Lana A. 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Ross Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers Mark Loeb , Dominik Mertz , Zain Chagla , John Conly Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers Trisha Greenhalgh , Danielle J. Cane , Matthew Oliver Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers Lisa M. Brosseau Metrics July 2023Volume 176, Issue 7KeywordsCOVID-19Health care providersInfectious diseasesPreventive medicine ePublished: 18 July 2023 Issue Published: July 2023 Copyright & PermissionsCopyright © 2023 by American College of Physicians. All Rights Reserved.PDF downloadLoading ...